Utilization of nanoindentation to examine bond line integrity in adhesively bonded composite structures

This study explores the probable correlation between degradation in bond line fracture toughness and the changes in the measurable mechanical properties of the adhesive layer due to environmental contamination in adhesively bonded composite structures. Nanoindentation technique is utilized to measure the adhesive Young's modulus and hardness. The proposed framework utilizes the large scale bridging of interfacial fracture proposed by Tvergaard and Hutchinson (Philos Mag A 70(4):641-656, 1994), for considering the significant contribution of plasticity on the macroscopic bond line fracture toughness. A typical adhesive-adherend material system (EA9394/Hexcel IM7-G/8552) exposed to different contaminants at the same concentration is examined. Hardness measurements showed two distinct effects of contamination on the adhesive material; namely (1) softening and (2) hardening depending on the class of contaminants. In addition, macroscopic mode-I fracture toughness is independently measured by double cantilever beam test. Finite element method employing cohesive zone method is used to rationalize the experimental results and the prospective scaling-laws. The combined experimental results of macroscopic properties and the numerical results of the interfacial properties showed the role of contamination in reducing the plastic dissipation within the bond line and the macroscopic deterioration of the bond line effective fracture toughness. The results showed the existence of a correlation similar to Tvergaard and Hutchinson's results. However, the results suggest a scaling between the interfacial cohesive fracture toughness and the measurable flow stress. The scaling correlation has the potential to be utilized in assessing the relative trend between different contaminants. While the proposed scaling is verified for a common adhesive-adherend system in aerospace industry, with additional examination of other systems, the proposed scaling law might facilitate the utilization of the perceived non-destructively evaluated indentation hardness to serve as an indicator for the bond line macroscopic fracture toughness.